Magnetism

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  • Created on: 02-08-17 18:07

Magnetism

4.7 Magnetism and electromagnetism

Electromagnetic effects are used in a wide variety of devices. Engineers make use of the fact that a magnet moving in a coil can produce electric current and also that when current flows around a magnet it can produce movement. It means that systems that involve control or communications can take full advantage of this.

4.7.1 Permanent and induced magnetism, magnetic forces and fields

4.7.1.1 Poles of a magnet

The poles of a magnet are the places where the magnetic forces are strongest. When two magnets are brought close together they exert a force on each other. Two like poles repel each other. Two, unlike poles, attract each other. Attraction and repulsion between two magnetic poles are examples of non-contact force. A permanent magnet produces its own magnetic field. An induced magnet is a material that becomes a magnet when it is placed in

a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly.

Students should be able to describe:

  • the attraction and repulsion between unlike and like poles for permanent magnets

  • the difference between permanent and induced magnets.

4.7.1.2 Magnetic fields

The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field. The force between a magnet and a magnetic material is always one of attraction. The strength of the magnetic field depends on the distance from the magnet. The field is strongest at the poles of the magnet. The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point. The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet. A magnetic compass contains a small bar magnet. The Earth has

a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.

Students should be able to:

  • describe how to plot the magnetic field pattern of a magnet using a compass

  • draw the magnetic field pattern of a bar magnet showing how strength and direction change from one point to another

  • explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic.

4.7.2 The motor effect

4.7.2.1 Electromagnetism

When a current flows through a conducting wire a magnetic field is produced around the wire. The strength of the magnetic field depends on the current through the wire and the distance from the wire. Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform. The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the strength of

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